In recent years, optical recording media such as read-only discs and optical recording discs have been heavily used as an information recording medium which records or stores a large amount of information such as motion picture. As one example, a high density optical disc (so-called Blu-ray Disc, hereinafter sometimes referred to as “Blu-Ray Disc”) which utilizes a laser beam at a wavelength of 400 nm has been proposed (Patent Document 1). The Blu-Ray Disc is an optical recording medium comprising a resin substrate, a recording and reading layer, a cover coat layer and a hard coat layer in this order. The hard coat layer is required to have abrasion resistance and hardness. On the other hand, a small deformation of the disc as a whole when the environmental temperature or the environmental humidity changes is required so that information can stably be read or write even when the environmental temperature or humidity changes.
Here, it is clear by studies by the present inventors that the deformation of the disc accompanying the environment temperature change is dominated by the elastic modulus of the cover coat layer. That is, the deformation of the disc accompanying the temperature change is significant when the elastic modulus of the cover coat layer is great. On the contrary, the deformation is small when the elastic modulus of the cover coat layer is small. Accordingly, the elastic modulus of the cover coat layer is preferably small in view of a small deformation accompanying the environment temperature change.
However, in general, the smaller the elastic modulus of the cover coat layer, the more the hardness of the hard coat layer formed on the cover coat layer decreases. Accordingly, in view of the hardness of the hard coat layer i.e. the abrasion resistance, the hardness, etc. of an optical recording medium, the elastic modulus of the cover coat layer is preferably great.
Further, it is estimated that the deformation of an optical recording medium accompanying the environment humidity change is dominated by the deformation accompanying absorption/desorption of water of the resin substrate. This is clear since the deformation of an optical recording medium comprising a resin substrate and a recording and reading layer alone is very large. Usually, the deformation can be reduced by further forming a cover coat layer and a hard coat layer on the recording and reading layer. It is considered that the deformation accompanying absorption/desorption of water of the cover coat layer and the hard coat layer offsets the deformation of the resin substrate present on the opposite side via the recording and reading layer, which is a relatively hard layer. It is considered that the degree of decreasing the deformation depends on a thick layer, and it mainly depends on the elastic modulus of the cover coat layer which is thicker than the hard coat layer in general. That is, an elastic modulus of the cover coat layer in a proper range (a relatively high elastic modulus) is favorable since the deformation of the disc accompanying the environmental humidity change is small. In a case where the elastic modulus of the cover coat layer is too low, the deformation accompanying absorption/desorption of water of the resin substrate by the environmental humidity change cannot sufficiently be suppressed and as a result, the deformation of the disc as a whole by the environmental humidity change is significant in some cases. Further, if the elastic modulus of the cover coat layer is too high, the deformation of the cover coat layer is more significant than the deformation of the resin substrate, and the deformation on the side opposite to the resin substrate is increased. Accordingly, in view of a small deformation accompanying the environmental humidity change, the elastic modulus of the cover coat layer is preferably within a proper range at a relatively high elastic modulus region.
With respect to the above-described relation between the elastic modulus of the cover coat layer and the required performance, in summary, (1) the cover coat layer preferably has a high elastic modulus in view of the surface hardness of the optical recording medium, (2) the cover coat layer preferably has a low elastic modulus in view of the deformation accompanying the environmental temperature change, and further, (3) the cover coat layer preferably has an elastic modulus within a proper range at a relatively high elastic modulus region in view of the deformation accompanying the environmental humidity change.
A material which undergoes a small deformation by the environmental temperature change (2) and the environmental humidity change (3) is proposed, for example, in Patent Document 1. The document discloses that a composition containing an urethane acrylate and another acrylate is suitable as a material for a light transmitting layer of an optical information medium, and according to studies by the present inventors, by use of the above material for a light transmitting layer, it may be possible to produce an optical recording medium of which the deformation accompanying the environmental temperature or humidity change is not so great.
Further, a material which has a high surface hardness (1) and of which the deformation by the environmental humidity change is small (3) is proposed, for example, in Patent Document 2.
Patent Document 1: JP-A-2003-263780
Patent Document 2: JP-A-2007-131698